Electrical measurements conducted through holes drilled in the ground play an important part when investigating soil quality. When considering for example disposal sites for nuclear waste, it is important to determine the rock structure, and in particular the amount and quality of clefts found therein. Another interesting application for these kinds of measurements is ore prospecting.
Known in prior art are various electrical measuring methods for investigating the soil. For instance, the so called single point resistance (SPR) is based on measuring the resistance between an electrode placed in a hole drilled in the ground and the surroundings of the hole. In case of rock, for instance, the resistance by a solid rock is high, but typically decreases significantly by rock fractures or clefts filled with water. The resistance can thus be used when investigating the positions of clefts or the like.
Another known method is mise á la masse. It involves feeding electric current into an electrode, which is placed in a hole drilled in the ground, and measuring the voltage from other holes drilled in the ground at a distance from the first hole. When the current electrode is placed in a conductive area, for example by a wet rock cleft or ore deposit, a nearly constant potential is formed in said area. The higher the resistance of the area outside the conductive area, the faster is the decrease in the potential outside the conductive area. Examining the voltage in and outside the conductive area allows thus the determination of geometry, and also, to some extent, of conductivity of the conductive area.
The methods described above have one significant problem. In order to ensure accurate measuring, the electrical measuring area should be limited near the electrode. For example in SPR, if the entire hole is filled with water, and is therefore conductive, the resistance for instance by a rock cleft does not necessarily differ much from the resistance between the electrode and the walls of the hole in other parts of the hole. In mise á la masse, for example when investigating the extent and shape of rock clefts, the current fed through an electrode may migrate both upwards and downwards in the conductive hole, and further to other clefts than the ones the investigation is focused on. In this case, when a conductive area, i.e. a cleft, is detected by measuring the voltage, it is not certain that the cleft in question is the one into which the current is fed. The resolution of these methods is therefore inadequate.